Waveguide-integrated passive components may be used to modify the transmission spectrum through a waveguide, such as a metallic slot waveguide. For instance, high Q-factor band-pass filters can be useful to filter a transmitted signal into different spectral bands in order to realize wavelength division multiplexing. Additionally, passive filters can be applied for routing surface plasmon polaritrons (SPPs) through a circuit of waveguides. Passive filters can play an important role for sensing applications too. For example, sharp plasmonic notch filters are useful for a future development of a surface-enhanced Raman scattering setup on chip.
Resonators can be made in a metal-insulator-metal (MIM) plasmonic waveguide that show strong resonance. In order to obtain such a strong resonance, Bragg reflectors or Bragg mirrors are typically used. The resonators are made by introducing a resonant cavity in a Bragg reflector inside a MIM plasmonic waveguide. Bragg reflectors based on MIM waveguides have been demonstrated experimentally recently.
The modulation is achieved by varying the dielectric thickness and may be studied numerically. Physically, in these waveguides the alternative variation in the thickness leads to periodic modulation of the mode index. For example, for a 100 nm thick SiO2 core symmetrically surrounded by two semi-infinite gold cladding layers, the mode index equals 1.84 at a free space wavelength of 800 nm. When the core thickness is decreased to 60 nm, the mode index rises to 2.06. This modal index difference is the key point to designing these Bragg reflectors inside MIM waveguides.
When a cavity is introduced within the Bragg reflector, a sharp resonance appears in the stop-band of the Bragg reflector. In silicon photonics, photonic bandgap microcavities are known to have a resonance with a very high quality-factor approaching, for example, up to 108. However, if one transfers this concept from a very low-loss dielectric waveguide to a high-confinement metallic waveguide, losses increase dramatically due to the field penetration inside the metal layers, causing high ohmic losses. Combined with previously developed active plasmonic components, sharp plasmonic band-pass filters and notch filters inside metallic waveguides can lead to microscale fluorescence biosensors or integrated detection of surface enhanced Raman scattering. The passive filters are integrated in MIM waveguides due to the good field confinement.